This invention relates to a method and apparatus for controlling acceleration and deceleration. More particularly, the invention relates to an acceleration-deceleration control method and apparatus well-suited for driving the movable element of a robot.
A numerical control device or robot control device may be adapted to control the table or tool of a machine tool or the movable element of a robot, such as a robot arm, to achieve positioning at desired points as well as movement along a desired path. The control device accomplishes this by performing an arithmetic pulse distribution operation based on a commanded amount of movement, thereby to produce so-called distributed pulses, and applying these pulses to a servo system to drive the servomotors of the machine tool or robot. In order to obtain good servomotor starting and stopping characteristics so that a servomotor may be started and stopped smoothly, it is common practice to provide the train of distributed pulses with an acceleration/deceleration characteristic. FIGS. 1 and 2 each show frequency characteristics wherein a train of distributed pulses is provided with an acceleration characteristic at the beginning of the pulse train and with a deceleration characteristic at the end of the pulse train. In FIG. 1 the acceleration/deceleration characteristic is linear, while in FIG. 2 the characteristic describes an exponential function.
The linear acceleration/deceleration characteristic is particularly effective for controlling a machine tool having a high degree of rigidity, wherein the movable element of the machine tool, such as a solid bed, reaches a constant speed in a short period of time. Owing to the abrupt change in the frequency of the distributed pulses at points P1, P2, P3, however, the movable element is likely to undergo vibration at these points. Such vibration poses problems when driving a movable element having little rigidity, such as the arm of a robot. It has therefore been proposed to use the acceleration/deceleration characteristic having the exponential function shown in FIG. 2 in order to eliminate the aforementioned vibration. With the latter characteristic, however, more time is required for positioning and vibration will still occur at point P4.
Thus the prior art exhibits disadvantages of the aforementioned kind and is not entirely satisfactory.